You might want to email the developer. I can't see any way to do it, even with 3rd party rooted apps.

Edit:

I just did a little test, because this bothered me. SMS doesn't do fonts - it's 7-bit blocks. So I added a signature (it's in script) and sent myself a text. The signature is there - in the same font as the rest of the message. The script font is only in the app, while you're typing the signature. (It shows in script on the message on the screen, but it sends as plain text.)